Assuming the middle plane will be a height field, do you want the normals from which each height is measured along to be from the top plane or the middle plane? Or will the heights be along the lines formed from going from the 2D parameterization (with each corner being going from [0,0] to [1,1]) of the top plane to the middle plane?

xernobyl, simply take every vertex of your camera-aligned regular grid, create a ray that passes through it, and intersect that ray with the horizontal plane of the ocean. you'll see that it all reduces to a few multiplications and one division.